Purpose and features of the AIS system
Denis Korablev
  • 05.11.2021
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Purpose and features of the AIS system

Recently, there has been a constant increase in the number of vehicles on the roads, the same is happening in the sea. The annual replenishment of the world civilian fleet is 3000 ships, the displacement of which is over 100 tons. And how fast the small fleet is growing, statistics are silent. Contemplating a huge number of masts in any Turkish marina, it is not a sin to assume that there are at least 4000 pieces per year.

The sea routes of large ships and small boats intersect more and more often; encounters do not always take place at a safe distance, and not all encounters end well. This problem is not solved today with the help of radar, although it detects the appearance of large ships near the yacht, and helps to understand the direction and approximate speed of movement.

At the beginning of the 21st century (2000), in order to reduce risky collisions among maritime transport, the Automatic Identification System (AIS) was developed, which allows obtaining reliable and detailed information about the existing navigation situation at a certain moment.

The experience of using the system turned out to be so successful that after 2 years AIS terminals, at the request of the International Maritime Organization, were obliged to install on cargo ships with a displacement of more than 300 tons, and on ships of any tonnage carrying passengers if they make international flights. According to experts 2yachts, in recent years, in addition to simple object recognition, AIS has significantly expanded its functions and is now increasingly called the "Automatic Information System" ...

Features of the AIS system

The ship module is an ultrashort wave digital transceiver that is linked to the ship's navigation system and set to automatic mode. Every 10 seconds (at a stop - every 3 minutes), it promptly reports information: unique MMSI ID, navigation status, exact coordinates, course and speed, exact time stamp and angular rate of turn.

Statistical information is reported every 6 minutes: name, type, size of the vessel, identifier with IMO number, radio call sign. The VHF transceiver is integrated with a positioning system (GLONASS, GPS, LORAN). In the same mode, data is transmitted along the route: destination, estimated time of arrival, type of cargo, draft, how many people are on board. If there is a danger threatening the safety of the vessel, then it is permissible to send messages from it in the form of texts that are entered manually.

This information can be viewed at the terminal as an information table for nearby maritime vehicles or as symbols that are overlaid on nautical charts in the chartplotter. Of course, this makes it much easier to figure out how to change the movement, given the location of the objects. In the system, information is exchanged by radio in the 162 MHz range (if we compare it with the radiation of a radar, the frequency is much lower). As you know, long radio waves bypass obstacles, for example, low islands or large ships. And the AIS range extends over 40 miles under favorable conditions, largely depending on how high the antenna is mounted.

Only a simpler version of the terminal (class "B") is allowed on yachts, as well as on other vessels with a carrying capacity of less than 300 tons. It has a lower transmitter power (only 2 watts, not 12.5 watts as in class "A" equipment), which allows you to receive messages within a radius of five miles. A simpler information dissemination algorithm allows sending messages only when there is a free space on the air between the radio traffic of large ships with class "A" equipment. On any of the 2 AIS channels at each aspect of time, only one block of digital readings can be transmitted, and class "A" equipment is able to agree in advance on the order of their issuance.

On the one hand, there is discrimination against small-sized fleets, on the other hand, being in a restless sea at night it is pleasant to know that on a tanker sailing not far off the watchman knows for sure that there is a 45-foot yacht next to him.

AIS offers another option for users: to install a receiver from which no messages can be sent, but you can see all the movements of ships equipped with fully functional terminals. Nowadays there is no need to purchase separate devices, as manufacturers (for example, Icom and Standard Horizon) install this function in the VHF radio. Everything seems to be fine, and not expensive, and compact, but looking at a text table on a small screen with a low resolution is a problem, and building at least a semblance of a map is generally out of the realm of fantasy.

Therefore, over time, AIS receivers have been developed that do not display graphical data; but they can convert the information into standard NMEA packets that most chartplotters understand. There are receivers that it is possible to connect to a computer monitor using USB, or transfer information via Wi-Fi to mobile devices that run on the iOS or Android platform.

AIS equipment has the same frequency range as the radio on board, so it is not necessary to install an additional antenna. But it should be borne in mind that two different devices must be connected to one antenna through a splitter, which usually leads to a decrease in the signal level. And if the only antenna suddenly fails, then the ship will immediately be deprived of 2 safety systems.

Conclusion: AIS is an indispensable assistant for helmsmen of maritime transport in operational maneuvering, and will be useful both on charter yacht, and on a small sea boat for fishing.

What other features does the system provide?

Using AIS, various shipping companies control the movements of their ships; government agencies may need location information of any cargo or vessel at any time. Consequently, AIS equipment is based on numerous coastal stations.

In emergency situations, AIS is used to search and organize rescue operations of mariners using emergency buoys, transmitting system data as a priority. Also, its accurate data is used in the investigation of accidents involving aviation. AIS enables national authorities to track and monitor their fishing fleets, effectively monitoring their activities.

They also release "virtual" buoys, they got their name because the actual location of objects does not always correspond to the coordinates indicated in the messages. Most often, transmitters are installed on the coastal strip in order to warn of dangers passing by ships, for example, rocks that are difficult to see, or headlands that do not have lighthouses and protrude far into the sea.

Surprisingly, AIS receivers are also placed on satellites! The radius of transmission of its signals on our planet is limited by the visibility of the horizon, but in outer space they can easily receive reception from several hundred kilometers. At least ten space devices monitoring the sea flow are operating around planet Earth.

Today, information about the movement of maritime transport, collected from coastal and satellite stations, are brought together and become available for a fee to everyone through service providers (one of them is Google Earth). The information is read-only, users are not visible on the AIS network.

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